1. Field of the Invention
The present invention relates to a motor driving technique.
2. Description of the Related Art
Brushless DC motors are employed in various usages such as spindle motors employed in optical disk devices, HDDs (hard disk drives), office automation equipment, fan motors, and so forth. Such a brushless DC motor has no brush commutation mechanism. Accordingly, with such a brushless DC motor, there is a need to switch the direction of a current to be supplied to a coil according to the position of a rotor. The driving method employed in such a brushless DC motor can be roughly classified into two methods, i.e., a sensor method using rotor position information (Hall signals) obtained from a Hall element or rotary encoder, and a sensorless method without involving such a Hall element in which the rotor position is estimated based on the zero-crossing point of the back electromotive force (inductive voltage) that occurs across the coil.
With the sensor method using a Hall element and an FG magnet, such an arrangement is capable of acquiring the position information not only when the rotor is in a rotating state, but also when the rotor is in a stationary state. However, with the sensorless method, the rotor position is estimated based on the back electromotive force that occurs according to the rotation of the rotor. This leads to a problem in that such an arrangement is not capable of detecting the rotor position with high precision when the motor is in a stationary state or otherwise when the motor is in a low-speed rotating state. In order to solve such a problem, a driving circuit employed with the sensorless method is provided with a function of detecting the position (which will be referred to as the “initial position”) of the rotor when the motor is started up.
As such a method for detecting the initial position of the rotor, an inductive sensing method has been proposed. Description thereof will be made as an example regarding a three-phase brushless motor. With such an example, a step voltage is applied across two phase electrodes from among three phase electrodes (U, V, W) while maintaining the rotor in a stationary state. The initial position of the rotor is detected based on the current that flows through the coil in this state. FIG. 1 is a waveform diagram for explaining an operation for detecting the initial position of the rotor using the inductive sensing method. For example, a positive step voltage VP is applied across the U-phase electrode and the V-phase electrode of the motor. In this state, measurement is performed for the period of time τU+ required for the coil current IU that flows through the coil to reach a predetermined threshold current +IP. Next, a negative step voltage VN is applied across the U-phase electrode and the V-phase electrode of the motor. In this state, measurement is performed for the period of time τU− required for the coil current IU that flows through the coil to reach a predetermined threshold current −IP. The difference ΔτU between the τU+ and τU− is calculated. The same measurement is performed for a pair of the V-phase electrode and the W-phase electrode and for a pair of the W-phase electrode and the U-phase electrode. As a result, the differences ΔτV and ΔτW are calculated. Subsequently, the initial position of the rotor is calculated based on the differences ΔτU, ΔτV, and ΔτW thus calculated.
As a result of investigating the inductive sensing method, the present inventors have come to recognize the following problem. Before the inductive sensing method is performed as shown in FIG. 1, there is a need to determine appropriate parameters (e.g., threshold currents ±IP). Specifically, the motor start-up test is performed for each of the initial positions, which are defined at a fine pitch. The parameters are adjusted for each of the initial positions. Specifically, the parameters that provide the highest motor start-up success rate are detected for each of the initial positions. Thus, the inductive sensing method requires a great number of executions of such a motor start-up test. The optimum parameters thus acquired are set for the driving circuit. This imposes a heavy burden on the designer of an electronic device that is to mount such a motor. In some cases, after the parameters to be used in the start-up sequence are optimized before shipping, the parameters thus determined can become unsuitable due to changes in characteristics of the motor with long-term use, leading to the potential for failure in the start-up operation.
Furthermore, as shown in FIG. 1, after the positive step voltage VP is applied, there is a need to stand by until the coil current IU becomes zero before the negative step voltage VN is applied. With conventional techniques, such an arrangement requires a long standby time with a sufficient margin, leading to a problem of a long start-up time.